A window assembly of this kind has been disclosed in U.S. Pat. No. 4,158,272, where a window frame forms a window opening. A guide channel extends along the frame above and below the frame. Two glass panels are directly fitted in the channel so as to be slidable, in order that the window opening is opened and closed by the panels. Accordingly, the window assembly of this kind needs only one such guide channel. Further, window glasses are used as the sliding panels. Hence, this window assembly is economical to manufacture. To make the window assembly watertight, weather strips are mounted between the window frame and the panes on each lateral side of the frame such that the strips bear on the panes in a watertight manner. Also, weather strips are fitted in the guide channel in such a way that they abut against the panes in a watertight manner. Frame members are mounted to opposite ends of the panes. A watertight member which bears on one of the frame members is mounted to at least the other frame member, to secure watertightness when the window opening is closed.
If the frame members were allowed to make close contact with the weather strips, the frame members would protrude beyond the surface of the panes because of the spaces in which they are mounted. Thus, the panes would experience a large resistance when slid, whereby the panes would become less easy to open and close. Therefore, in order to avoid this undesirable situation, the end portions of the frame members and of the watertight member are located outside the weather strips, creating air gaps between the end portions of the frame members and of the watertight member and the weather strips. This prevents the problem that rainwater, dust, or other foreign matter intrudes into the vehicle through the gaps when the window opening is closed.